FIG. 1 shows one example of a conventional drilling system for drilling an earth formation. The drilling system includes a drilling rig 10 used to turn a drilling tool assembly 12 that extends downward into a well bore 14. The drilling tool assembly 12 includes a drilling string 16, and a bottomhole assembly (BHA) 18, which is attached to the distal end of the drill string 16. The “distal end” of the drill string is the end furthest from the drilling rig.
The drill string 16 includes several joints of drill pipe 16a connected end to end through tool joints 16b. The drill string 16 is used to transmit drilling fluid (through its hollow core) and to transmit rotational power from the drill rig 10 to the BHA 18. In some cases the drill string 16 further includes additional components such as subs, pup joints, etc.
The BHA 18 includes at least a drill bit 20. Typical BHA's may also include additional components attached between the drill string 16 and the drill bit 20. Examples of additional BHA components include drill collars, stabilizers, measurement-while-drilling (MWD) tools, logging-while-drilling (LWD) tools, subs, hole enlargement devices (e.g., hole openers and reamers), jars, accelerators, thrusters, downhole motors, and rotary steerable systems.
In general, drilling tool assemblies 12 may include other drilling components and accessories, such as special valves, such as kelly cocks, blowout preventers, and safety valves. Additional components included in a drilling tool assembly 12 may be considered a part of the drill string 16 or a part of the BHA 18 depending on their locations in the drilling tool assembly 12.
The drill bit 20 in the BHA 18 may be any type of drill bit suitable for drilling earth formation. Two common types of drill bits used for drilling earth formations are fixed-cutter (or fixed-head) bits and roller cone bits. FIG. 2 shows one example of a fixed-cutter bit. FIG. 3 shows one example of a roller cone bit.
Referring to FIG. 2, fixed-cutter bits (also called drag bits) 21 typically comprise a bit body 22 having a threaded connection at one end 24 and a cutting head 26 formed at the other end. The head 26 of the fixed-cutter bit 21 typically includes a plurality of ribs or blades 28 arranged about the rotational axis of the drill bit and extending radially outward from the bit body 22. Cutting elements 29 are embedded in the raised ribs 28 to cut formation as the drill bit is rotated on a bottom surface of a well bore. Cutting elements 29 of fixed-cutter bits typically comprise polycrystalline diamond compacts (PDC) or specially manufactured diamond cutters. These drill bits are also referred to as PDC bits.
Referring to FIG. 3, roller cone bits 30 typically comprise a bit body 32 having a threaded connection at one end 34 and one or more legs (typically three) extending from the other end. A roller cone 36 is mounted on each leg and is able to rotate with respect to the bit body 32. On each cone 36 of the drill bit 30 are a plurality of cutting elements 38, typically arranged in rows about the surface of the cone 36 to contact and cut through formation encountered by the drill bit. Roller cone bits 30 are designed such that as a drill bit rotates, the cones 36 of the roller cone bit 30 roll on the bottom surface of the well bore (called the “bottomhole”) and the cutting elements 38 scrape and crush the formation beneath them. In some cases, the cutting elements 38 on the roller cone bit 30 comprise milled steel teeth formed on the surface of the cones 36. In other cases, the cutting elements 38 comprise inserts embedded in the cones. Typically, these inserts are tungsten carbide inserts or polycrystalline diamond compacts. In some cases hardfacing is applied to the surface of the cutting elements and/or cones to improve wear resistance of the cutting structure.
For a drill bit 20 to drill through formation, sufficient rotational moment and axial force must be applied to the drill bit 20 to cause the cutting elements of the drill bit 20 to cut into and/or crush formation as the drill bit is rotated. The axial force applied on the drill bit 20 is typically referred to as the “weight on bit” (WOB). The rotational moment applied to the drilling tool assembly 12 at the drill rig 10 (usually by a rotary table or a top drive mechanism) to turn the drilling tool assembly 12 is referred to as the “rotary torque”. The speed at which the rotary table rotates the drilling tool assembly 12,. typically measured in revolutions per minute (RPM), is referred to as the “rotary speed”. Additionally, the portion of the weight of the drilling tool assembly supported at the rig 10 by the suspending mechanism (or hook) is typically referred to as the hook load.
During drilling, the actual WOB is not constant. Some of the fluctuation in the force applied to the drill bit may be the result of the drill bit contacting with formation having harder and softer portions that break unevenly. However, in most cases, the majority of the fluctuation in the WOB can be attributed to drilling tool assembly vibrations. Drilling tool assemblies can extend more than a mile in length while being less than a foot in diameter. As a result, these assemblies are relatively flexible along their length and may vibrate when driven rotationally by the rotary table. Drilling tool assembly vibrations may also result from vibration of the drill bit during drilling. Several modes of vibration are possible for drilling tool assemblies. In general, drilling tool assemblies may experience torsional, axial, and lateral vibrations. Although partial damping of vibration may result due to viscosity of drilling fluid, friction of the drill pipe rubbing against the wall of the well bore, energy absorbed in drilling the formation, and drilling tool assembly impacting with well bore wall, these sources of damping are typically not enough to suppress vibrations completely.
Vibrations of a drilling tool assembly are difficult to predict because different forces may combine to produce the various modes of vibration, and models for simulating the response of an entire drilling tool assembly including a drill bit interacting with formation in a drilling environment have not been available. Drilling tool assembly vibrations are generally undesirable, not only because they are difficult to predict, but also because the vibrations can significantly affect the instantaneous force applied on the drill bit. This can result in the drill bit not operating as expected. For example, vibrations can result in off-centered drilling, slower rates of penetration, excessive wear of the cutting elements, or premature failure of the cutting elements and the drill bit. Lateral vibration of the drilling tool assembly may be a result of radial force imbalances, mass imbalance, and drill bit/formation interaction, among other things. Lateral vibration results in poor drilling tool assembly performance, overgage hole drilling, out-of-round, or “lobed” well bores and premature failure of both the cutting elements and drill bit bearings.
When the drill bit wears out or breaks during drilling, the entire drilling tool assembly must be lifted out of the well bore section-by-section and disassembled in an operation called a “pipe trip”. In this operation, a heavy hoist is required to pull the drilling tool assembly out of the well bore in stages so that each stand of pipe (typically pipe sections of about 90 feet) can be unscrewed and racked for the later re-assembly. Because the length of a drilling tool assembly may extend for more than a mile, pipe trips can take several hours and can pose a significant expense to the well bore operator and drilling budget. Therefore, the ability to design drilling tool assemblies which have increased durability and longevity, for example, by minimizing the wear on the drilling tool assembly due to vibrations, is very important and greatly desired to minimize pipe trips out of the well bore and to more accurately predict the resulting geometry of the well bore drilled.
Many companies offer drilling services for the purposes of improving drilling performance. These services typically include modeling up to around 200 feet of the BHA with representative factors assumed for the influence of the drill string and the drill bit during drilling. The drill string is typically modeled as a spring and the spring constant assumed based on the expected configuration of the drill string. The BHA is typically modeled as a beam suspended from the spring at one end and excited by an excitation at the other end assumed to represent the excitation resulting from a drill bit interacting with the formation.
While prior art simulation methods, such as those described above provide a general means for predicting drilling tool assembly dynamics, simulation techniques have not been developed to cover actual drilling with a drilling tool assembly in a well bore including a complete simulation of the drill string, the BHA, and the drill bit that takes into account the interaction of the cutting elements on the drill bit with the earth formation being drilled. As a result, accurately modeling and predicting the response of a drilling tool assembly during drilling has been virtually impossible. Additionally, the change in the dynamic response of a drilling tool assembly while drilling when a component of the drilling tool assembly is changed has not been well understood.
Prior art drill bit simulation methods have been developed and used for the design or selection of drill bits independent of the drilling tool assemblies with which the drill bits will be used. As a result, optimized drill bit selection and design is typically an iterative process, which requires the collection and evaluation of field performance data obtained from many field runs using a selected drill bit. When a trend of drilling problems is found to occur for a particular bit, such as low rate of penetration or premature drill bit failure, a new drill bit may be selected or an adjustment made to the current bit design in hopes of obtaining better drilling performance in future runs. A design change or selection of a new drill bit is made independent of the drilling tool assembly with which the drill bit will be used, and many field runs with the new bit may occur before the actual drilling performance of the new drill bit can be confirmed. Similar iterative methods are used to determine an optimum or preferred selection of components in a drilling tool assembly. Such iterative design and selection methods are time consuming and can be costly for drilling operations. In particular, replacement of a poorly performing drill bit or failure of another component of a drilling tool assembly requires the time and expense of removing the drilling tool assembly from the well bore, which may take many hours depending on the depth of the well. Also, in many cases, after using several different drill bit designs in an attempt to improve drilling performance in a series of wells, it may later be determined that drilling problems may have been better corrected by changing other parameters of the drilling tool assembly, such as operating parameters for drilling or the make up of the BHA to avoid or minimize vibration modes of the drilling tool assembly during drilling.